Known electrical heating devices which, after reaching a critical temperature, rapidly decrease their output without the help of thermostatic regulation are based on two or more conductors and an intermediate resistance material, the resistivity of which starts to increase steeply at the critical temperature. Such materials are called PTC-materials (Positive Temperature Coefficient).
Known PTC-materials for self-limiting heating devices consist of crystalline polymers with conducting particles distributed therein. The polymers can be thermoplastic or crosslinked. In U.S. Pat. No. 3,243,753 the steep increase of the resistivity is explained by the expansion of the polymer leading to interruption of the contact between the conducting particles. In U.S. Pat. No. 3,673,121 the PTC effect is claimed to be due to phase changes of crystalline polymers with narrow molecular weight distribution.
According to J. Meyer, Polymer Engineering and Science, Nov. 1973, 462-468, the effect is explained by an alteration of the conductivity of the crystallites at the critical temperature.
Common for the known PTC-materials is that the resistivity alone is changed greatly above the critical temperature while the other physical properties generally remain unchanged. The temperature range in which the resistivity increases by a power of 10 is usually 50.degree.-100.degree. C. However, for many applications it is not satisfactory that the reduction of the power per degree is so small and that it is not possible to freely choose the temperature interval for the steep increase of the resistivity.
In an article by F. Bueche in J. of Applied Physics, Vol. 44, No. 1, January 1973, 532-533, it is described how, by combining several percent by volume of conducting particles in a semicrystalline matrix, a highly temperature-dependant resistivity is obtained. This resistivity is changed considerably in a small temperature interval around the crystal melting temperature. As the non-conducting matrix various hydrocarbon waxes are used. According to the article, it is also possible to add so-called "mechanical stabilizers", consisting of polymers soluble in the wax, whereby for obtaining good results, it is stated to be important that the wax and the polymer are soluble in each other, which means that only one phase may exist.